Part 8

Jasper (impure opaque or subtranslucent quartz).—Good green, yellow, red, and brown jasper has been found in the gravels of the Rio Grande at all of the localities that produce agate. The colors are quite vivid, and the material takes a fine polish. Some pieces of orbicular jasper (jasper with circular or eye-like markings) have been found in this material. These gravels commonly contain jasper as fragments that are suspended in clear chalcedony; this is called brecciated jasper and yields very handsome cabochons.

Many of the west Texas agate localities also produce jasper in quantity. Good jasper has been reported from north of Brackettville, Kinney County. Jasper is a minor constituent of the stream gravels in many parts of the State.

Sanidine

Composition: KAlSi₃O₈; commonly contains some sodium. Crystal system: monoclinic. Hardness: 6. Specific gravity: 2.57 to 2.58. Luster: vitreous to pearly. Color: colorless, white, pale yellow, and gray. Streak: uncolored. Cleavage: three directions. Fracture: conchoidal to uneven. Tenacity: brittle. Diaphaneity: transparent to subtranslucent. Refractive index: 1.52 to 1.53.

Some feldspars, including sanidine, show a nice blue sheen in reflected light parallel to certain crystallographic directions. Stones having this property are called moonstone. A clear yellowish sanidine showing an attractive blue sheen has been found in Brewster, Jeff Davis, and Presidio counties. The individual pieces are small, the average size being about one-eighth inch. The sanidine is found loose in the soil at some localities where it has weathered out of rhyolite, and specimens of the sanidine in the parent rock are not difficult to obtain. Very small cabochons can be cut from this material, but few lapidaries have done so because inexpensive larger pieces of moonstone can be obtained easily from foreign sources. However, the west Texas sanidine does show a blue sheen when cut and polished.

Spinel

Composition: MgAl₂O₄ (magnesium may be replaced in part by ferrous iron or manganese and the aluminum by ferric iron and chromium). Crystal system: isometric. Hardness: 8. Specific gravity: 3.5 to 4.1. Luster: vitreous to sub-metallic. Color: black, pink, red, blue, green, yellow, brown, and violet. Streak: white. Cleavage: one direction, imperfect. Fracture: conchoidal. Tenacity: brittle. Diaphaneity: transparent to opaque. Refractive index: variable, approximately 1.72 to 2.00.

In many areas of the world, fine quality, beautifully colored, transparent spinels are found and used as gems. The only gem-quality spinel reported thus far in Texas is black and opaque. Near Eagle Flat in Hudspeth County, black spinel crystals have been found associated with augite and natural glass; these minerals are weathering out of an intrusive igneous rock. The spinel crystals have an octahedral form which is common for this mineral (fig. 17). Most of the spinels are free of flaws, but because of their black color they have little value as gems. The crystals are found loose in the sand of streams near the outcrops of the igneous rock or embedded in the rock. They seldom exceed half an inch in diameter. These stones are primarily sought by collectors.

Tektite (Bediasite)

Composition: A natural glass, approximately 75% SiO₂, 15% Al₂O₃, 4% FeO, also MgO, Na₂O, K₂O, and traces of other elements. Crystal structure: amorphous. Hardness: 5 to 6. Specific gravity: 2.33 to 2.44. Luster: vitreous, often dull on weathered surfaces. Color: dark brown, greenish brown, appears black in thick sections. Streak: uncolored. Cleavage: none. Fracture: conchoidal. Tenacity: brittle. Diaphaneity: transparent to subtransparent. Refractive index: 1.488 to 1.512.

The average bediasite size is about 1 inch in diameter, although specimens approximately 3 inches in diameter are known. The uncut tektites are very interesting, showing a variety of shapes and surface features (Pl. V, A) and many exhibit contorted flow structure. The surface of many tektites is grooved or furrowed, while on others it is smooth or frosted. The Texas tektites are known as “bediasites,” after place names in Grimes County traceable to the Bedias Indians who formerly lived there.

Dark brown and greenish-brown tektites have been found in Texas in gravels at scattered localities in Walker, Grimes, Brazos, Burleson, Lee, Fayette, Gonzales, Lavaca, and DeWitt counties. Outside of Texas the only other authenticated tektite localities in the United States at the present time are in Dodge and Irwin counties, Georgia. A fragment of a similar tektite has recently been reported from near Martha’s Vineyard, Massachusetts. The tektites reported from Oklahoma are now known to be pebbles of obsidian.

Although tektites have little value or beauty as gemstones, they have been cut by lapidaries as both faceted and cabochon stones. Tektites take a high polish but are mostly so dark in color that they appear black.

The origin of tektites is of great scientific interest and is currently the subject of much debate. Some scientists believe that tektites are of meteoritic origin, while others believe that tektites were formed by various terrestrial processes. Since no one has actually observed a tektite to fall or form, and many of the theories of origin are difficult to prove without direct observation, the origin of tektites is likely to remain in controversy for some time.

Topaz

Composition: Al₂(F, OH)₂SiO₄. Crystal system: orthorhombic. Hardness: 8. Specific gravity: 3.4 to 3.6. Luster: vitreous. Color: pale blue, sky blue, greenish, white, wine yellow, straw yellow, grayish, pink, reddish, and orange. Streak: uncolored. Cleavage: one direction, basal, highly perfect. Fracture: conchoidal to uneven. Tenacity: brittle. Diaphaneity: transparent to subtranslucent. Refractive index: about 1.60 to 1.63. Dispersion: moderate.

Various yellow and smoky colored quartz gems are offered for sale as “Spanish Topaz,” “Smoky Topaz,” “Madeira Topaz,” and “Topaz Quartz.” These names are entirely misleading and should be dropped from usage.

Fine gem-quality white, pale-blue, and sky-blue topaz has been found near Streeter, Grit, and Katemcy, Mason County. This Texas gem material compares favorably in color, size, and clarity with topaz found anywhere in the United States. Fine crystals of topaz (Pl. V, B, and fig. 18) occasionally are found in pegmatite dikes associated with quartz, black tourmaline, cassiterite, and pink microcline. Many of the gem-bearing pegmatites have been eroded away, leaving the topaz concentrated in the stream beds. The stones mostly occur as frosted, stream-worn pebbles (Pl. VI, A) in the numerous small creeks in the area. The topaz is heavier than the quartz and microcline that compose the stream gravel and is commonly found immediately on top of the granite bed-rock in the bottom of the stream bed. The stones tend to lodge behind boulders or small dikes cutting across the stream.

The white or colorless stones are by far the most common, outnumbering the bluish stones about ten to one. The color of the blue stones tends to be irregularly distributed in zones parallel to the crystal faces. Topaz that is colored in this manner should be cut with the best blue color near the bottom or culet of the gem (fig. 19). If done correctly, this will give the entire gemstone the desirable blue color.

COLORLESS BLUE

The colorless stones can be turned pale yellow, yellowish brown, or straw yellow by exposure to X-ray radiation, and some of the bluish stones will fluoresce faintly yellowish under ultra-violet light.

The largest gem-quality topaz crystal yet found in North America has come from Mason County. It is a pale-blue crystal weighing 1,296 grams, now in the collection of the U.S. National Museum. Several other large pieces, some weighing over a pound, have been found. One large crystal, exact weight unknown, was found near Katemcy. Several gem cutters have estimated that this stone could easily yield a single, flawless pale-blue gem of about 500 carats. Many large gems have been cut from topaz found in this area, including at least one stone of over 300 carats.

One obstacle in the cutting of topaz is its perfect basal cleavage. The gemstone should be oriented so that no facet of the stone will be parallel to or within less than about 5 degrees of the cleavage direction, or the facet may be very difficult or impossible to polish.

It is difficult to estimate the productivity of this area since its discovery in the early 1900’s. Few systematic attempts have been made to exploit the deposits, and a great amount of the topaz thus far recovered has been found by private collectors. The Mason County topaz deposits are still very productive, and additional exploration may uncover even more gem-producing areas.

Topaz has also been found in stream gravels or pegmatites in Burnet, Llano, Gillespie, and El Paso counties but very rarely in gem quality.

Tourmaline

Composition: H₉Al₃(B·OH)₂Si₄O₁₉; hydrogen often replaced by iron, magnesium, calcium, or fluorine. Crystal system: hexagonal. Hardness: 7 to 7.5. Specific gravity: 2.98 to 3.20. Luster: vitreous to resinous. Color: black, brownish black, brown, blue, green, red, pink, yellow, and gray. Streak: uncolored. Cleavage: two directions, very imperfect. Fracture: subconchoidal to uneven. Tenacity: brittle. Diaphaneity: transparent to opaque. Refractive index: about 1.62 to 1.64.

Black tourmaline is schorl; brown tourmaline, dravite.